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I did in fact create a file in /usr/share/X11/xorg.conf.d/ named 20-radeon.conf and set up the swapbufferswait to false and then I added the vblank_mode 0 in /etc/drirc, and I'm getting full FPS in the game engines! Thanks a ton!

MSAA adds another processing step to the rendering pipeline: the resolve. This process combines subsamples to one sample for each pixel. This results in a constant performance hit. Beyond that, GPUs use various schemes to compress depth, stencil and color buffers, so that increasing sample depth does not result in much increase in memory bandwidth (after all, only the geometry edges can be different, GPUs take advantage of that) in typical cases. Therefore, performance is often quite similar for the different MSAA levels.

I would have expected the performance hit to be linear across all levels then. What warrants the change in performance from 4x to 6x in OpenArena 0.8.5 (though not in 0.8.8) and World of Padman?

I don't think that's the issue, since Michael has previously mentioned the fact that Valve was going to add the possibility to run pre-defined demos (scripted sequences) from the command prompt. That's what he needs for being able to automatically run the benchmarks. Doom 3 is used every now and then, but that is open sourced so I guess that's not a good example.

Because Larabel is too lazy to run them manually like every other review site on earth. I keep saying it but every Larabel apologist always comes to his rescue because they can't envision a better review site, they can only accept what Larabel gives them as an inalienable truth of the universe, all nay sayers be damned to the pits of hell for eternity and a day...

I would have expected the performance hit to be linear across all levels then.

The overhead really depends on where your bottlenecks are, how much geometry edges are renderered and the specific MSAA configuration. Newer Radeon GPUs (Cayman and up) use mixed MSAA/CSAA for high AA levels, for instance.

What warrants the change in performance from 4x to 6x in OpenArena 0.8.5 (though not in 0.8.8) and World of Padman?

6x doesn't exist. r600g only supports 2x, 4x and 8x. In practice, the driver upgrades to 8x if 6x is requested.

Edit: timing of these benchmarks was bad as well, recently a gallium optimization was implemented which may noticeably reduce performance impact of MSAA.